Module 4 Section 5

Question 1

Describe the muscle receptors and their function.

Answer 1

Muscle spindles

• Monitors changes in muscle length and plays a key role in stretch reflexes
• Distributed throughout the muscle as a collection of specialized muscle cells known as intrafusal fibres. Intrafusal fibres lie within spindle shaped connective tissue and are found among the extrafusal fibres. The ends of the intrafusal fibre are contractible but the middle region is not
• Intrafusal fibres are innervated by alpha motor neurons, muscle spindles are innervated by gamma motor neurons
• The central region of a muscle spindle contains sensory afferent fibres that are activated by stretch and transmit info on muscle length and rate of stretch to the CNS

Golgi tendon organs

• Primary purpose = respond to changes in muscle tension
• They’re receptors found at the junction of tendons and muscle fibres, and respond to both stretch and contraction of the muscle
• Extrafusal fibres contract -> tension pulls on the tendons -> stretch activates the afferent fibres intertwined within the tendons
• The stronger the pull on the tendon, the higher the rate of firing of the golgi tendon organ afferents
• Info -> brain (processing)
• Most info = subconscious (you’re aware of tension in your muscles, but not of their length)

Question 2

Describe the 3 levels of neural input involved in motor control.

Answer 2

Afferent neurons

• Generally occurs at the level of the spinal cord and are involved in spinal reflexes
• Essential for maintaining posture and protective movements (withdrawal reflex)

Primary motor cortex

• Fibres originating from pyramidal cells within the primary motor cortex descend and terminate on motor neurons or the interneuron that terminate on motor neurons in the spinal cord
• These fibres = corticospinal motor system
• The system mediates fine voluntary movements of body parts such as the hands and fingers

Brain stem

• Also called the multineuronal motor system
• Influenced by motor regions of the cortex, the cerebellum, and the basal nuclei
• The system regulates overall body posture and involuntary movements of the larger muscle groups

Question 3

Match the corresponding vocabulary to match the definition:

• proprioception
• golgi tendon organs
• spinal reflexes
• muscle spindles
• corticospinal motor system
• extrafusal fibres
• intrafusal fibres

1) ___: Your awareness of your body in the environment
2) ___: A collection of specialized muscle cells in which muscle spindles are distributed
3) ___: The muscle receptor responsible for responding to muscle tension
4) ___: These are essential for maintaining posture and protective movements such as the withdrawal reflex
5) ____: The system that mediates fine voluntary movements of body parts such as the hands and fingers
6) ___: Regular muscle fibres
7) ___: The muscle receptor responsible for responding to stretch

Answer 3

1) proprioception: Your awareness of your body in the environment
2) intrafusal fibres: A collection of specialized muscle cells in which muscle spindles are distributed
3) golgi tendon organs: The muscle receptor responsible for responding to muscle tension
4) spinal reflexes: These are essential for maintaining posture and protective movements such as the withdrawal reflex
5) corticospinal motor system: The system that mediates fine voluntary movements of body parts such as the hands and fingers
6) extrafusal fibres: Regular muscle fibres
7) muscle spindles: The muscle receptor responsible for responding to stretch

Question 4

Describe how damage to the following areas would affect movement: brainstem nuclei and muscle spindle.

Answer 4

Brainstem nuclei:
- Damage would result in decreased input on motor neurons which are responsible for the excitation of muscle fibres. With decreased input on the motor neurons, voluntary movement would be hindered.

Muscle spindle:
- Damage to the peripheral receptor, such as a muscle spindle receptor, would affect the detection of muscle length. This would hinder the ability of the afferent nerves to properly convey info to the brainstem and the primary motor cortex. Damage to muscle spindles would hinder the brain’s ability to coordinate purposeful muscle activity.